368 CHAPTER 30: BGP CONFIGURATION
■ Unfeasible Routes Length: The total length of the Withdrawn Routes field in
bytes. A value of 0 indicates neither any route is being withdrawn from service,
nor Withdrawn Routes field is present in this Update message.
■ Withdrawn Routes: This is a variable length field that contains a list of IP
prefixes of routes that are being withdrawn from service.
■ Total Path Attribute Length: Total length of the Path Attributes field in bytes. A
value of 0 indicates that no Network Layer Reachability Information field is
present in this Update message.
■ Path Attributes: List of path attributes related to NLRI. Each path attribute is a
triple <attribute type, attribute length, attribute value> of variable length. BGP
uses these attributes to avoid routing loops, perform routing and protocol
extension.
■ NLRI (Network Layer Reachability Information): Reachability information is
encoded as one or more 2-tuples of the form <length, prefix>.
Notification
A Notification message is sent when an error is detected. The BGP connection is
closed immediately after sending it. Notification message format is shown below:
Figure 118 BGP Notification message format
■ Error Code: Type of Notification.
■ Error Subcode: Specific information about the nature of the reported error.
■ Data: Used to diagnose the reason for the Notification. The contents of the
Data field depend upon the Error Code and Error Subcode. Erroneous part of
data is recorded. The Data field length is variable.
Keepalive
Keepalive messages are sent between peers to maintain connectivity. Its format
contains only the message header.
Route-refresh
A route-refresh message is sent to a peer to request the resending of the specified
address family routing information. Its format is shown below:
Figure 119 BGP Route-refresh message format
AFI: Address Family Identifier.
Res: Reserved. Set to 0.
SAFI: Subsequent Address Family Identifier.
0 7 15 31
Data
Error Code Error SubCode
015 31
AFI Res. SAFI
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